Max Planck Institute of Quantum Optics
Garching, Germany

The Max-Planck-Institute of Quantum Optics is a part of the Max Planck Society which operates 87 research facilities in Germany.The institute is located in Garching, Germany, which in turn is located 10 km north-east of Munich. Five research groups work in the fields of attosecond physics, laser physics, quantum information theory, laser spectroscopy, quantum dynamics and quantum many body systems. Wikipedia.

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The probability to find a certain number of photons inside a laser pulse usually corresponds to a classical distribution of independent events, the so-called Poisson-distribution. There are, however, light sources with non-classical photon number distributions that can only be described by the laws of quantum mechanics. A well-known example is the single-photon source that may find application in quantum cryptography for secret key distribution or in quantum networks for connecting quantum memories and processors. However, for many applications in nonlinear quantum optics light pulses with a certain fixed number of photons, e.g. two, three or four, are highly desirable. A team of scientists from the Quantum Dynamics Division of Professor Gerhard Rempe at the Max Planck Institute of Quantum Optics (Garching near Munich) has now succeeded to make the first steps in this direction. Using a strongly coupled atom-cavity system, they were the first to observe the so-called two-photon blockade: the system emits at most two photons at the same time since its storage capacity is limited to that number (PRL, 31 March 2017). A naive approach for generating a stream of single photons would be to sufficiently attenuate the intensity of a laser beam. But in this case the number of photons still varies from pulse to pulse, and only when averaging over many pulses a mean photon number of one is observed. Applications instead require a fixed number of exactly one photon per pulse. The fluctuations of the photon number per pulse can be strongly reduced by using a single atom as a single-photon source. When the atom is illuminated by a laser beam, it can absorb only one photon at a time, thereby making a transition from the ground state to an excited state. A second photon can only be absorbed after the atom has fallen back to the ground state by emitting a photon. Therefore, no more than one photon is detected in the emitted light field at the same time, an effect that is known as "single-photon blockade". In order to extend this principle to a "two-photon blockade" one has to go beyond a single atom and look for a system that can store more than one photon, but not more than two. To this end, the MPQ physicists combine the single atom with a cavity that provides additional storage capacities. A cavity can absorb an unlimited number of photons and exhibits a correspondingly large number of energy states that lie – similar to a "ladder" – in exactly the same distance from each other. Inserting a single atom into the cavity introduces a nonlinear element. This causes the energy levels to split by a different amount for each of the 'ladder steps'. Hence, laser light can excite the system only up to the level to which it is tuned to. The number of photons that can be stored is thus limited to a certain number, and therefore, not more photons than that can be emitted. In the experiment, the physicists hold a single rubidium atom in an optical trap inside a cavity made of two high-finesse mirrors. The frequency of the incoming laser beam is tuned to an energy level requiring the absorption of two photons for its excitation. During the five seconds of atom storage time around 5000 measurement cycles are carried out, during which the system is irradiated by a probe laser and emission from the cavity is recorded via single-photon detectors. "Interestingly, the fluctuations in the number of emitted photons does strongly depend on whether we excite the cavity or the atom," points out the project leader Dr. Tatjana Wilk. "The effect that the absorption of two photons suppresses further absorption leading to emission of two or less photons is only achieved in case of atomic excitation. This quantum effect does not appear when we excite the cavity. In this case, we observe an enhanced signal of three and more photons per light pulse." Christoph Hamsen, doctoral candidate at the experiment, explains the underlying processes: "When the atom is excited we are dealing with the interplay between two conflicting mechanisms. On the one hand, the atom can absorb only one photon at a time. On the other hand, the strongly coupled atom-cavity system is resonant with a two-photon transition. This interplay leads to a sequence of light pluses with a non-classical photon distribution." And Nicolas Tolazzi, another doctoral candidate, adds: "We were able to observe this behaviour in correlations between detected photons where the coincidence of three photons was significantly suppressed compared to the expectation for the classical case." Prof. Gerhard Rempe gives an outlook on possible extensions of the experiment: "At present, our system emits light pulses with two photons at maximum, but also pulses with fewer, one or even zero, photons. It acts like a kind of 'low pass'. There are, however, a number of applications for quantum communicating and quantum information processing where exactly two, three or four photons are required. Our ultimate goal is the generation of pure states where each light pulse contains exactly the same desired number of photons. The two-photon blockade demonstrated in our experiment is the first step in this direction." Olivia Meyer-Streng More information: Christoph Hamsen et al. Two-Photon Blockade in an Atom-Driven Cavity QED System, Physical Review Letters (2017). DOI: 10.1103/PhysRevLett.118.133604

News Article | May 25, 2017

An international team of physicists has monitored the scattering behavior of electrons in a non-conducting material in real-time. Their insights could be beneficial for radiotherapy. We can refer to electrons in non-conducting materials as 'sluggish'. Typically, they remain fixed in a location, deep inside an atomic composite. It is hence relatively still in a dielectric crystal lattice. This idyll has now been heavily shaken up by a team of physicists led by Matthias Kling, the leader of the Ultrafast Nanophotonics group in the Department of Physics at Ludwig-Maximilians-Universitaet (LMU) in Munich, and various research institutions, including the Max Planck Institute of Quantum Optics (MPQ), the Institute of Photonics and Nanotechnologies (IFN-CNR) in Milan, the Institute of Physics at the University of Rostock, the Max Born Institute (MBI), the Center for Free-Electron Laser Science (CFEL) and the University of Hamburg. For the first time, these researchers managed to directly observe the interaction of light and electrons in a dielectric, a non-conducting material, on timescales of attoseconds (billionths of a billionth of a second). The study was published in the latest issue of the journal Nature Physics. The scientists beamed light flashes lasting only a few hundred attoseconds onto 50 nanometer thick glass particles, which released electrons inside the material. Simultaneously, they irradiated the glass particles with an intense light field, which interacted with the electrons for a few femtoseconds (millionths of a billionth of a second), causing them to oscillate. This resulted, generally, in two different reactions by the electrons. First, they started to move, then collided with atoms within the particle, either elastically or inelastically. Because of the dense crystal lattice, the electrons could move freely between each of the interactions for only a few ångstrom (10-10 meter). "Analogous to billiard, the energy of electrons is conserved in an elastic collision, while their direction can change. For inelastic collisions, atoms are excited and part of the kinetic energy is lost. In our experiments, this energy loss leads to a depletion of the electron signal that we can measure," explains Professor Francesca Calegari (CNR-IFN Milan and CFEL/University of Hamburg). Since chance decides whether a collision occurs elastically or inelastically, with time inelastic collisions will eventually take place, reducing the number of electrons that scattered only elastically. Employing precise measurements of the electrons' oscillations within the intense light field, the researchers managed to find out that it takes about 150 attoseconds on average until elastically colliding electrons leave the nanoparticle. "Based on our newly developed theoretical model we could extract an inelastic collision time of 370 attoseconds from the measured time delay. This enabled us to clock this process for the first time," describes Professor Thomas Fennel from the University of Rostock and Berlin's Max Born Institute in his analysis of the data. The researchers' findings could benefit medical applications. With these worldwide first ultrafast measurements of electron motions inside non-conducting materials, they have obtained important insight into the interaction of radiation with matter, which shares similarities with human tissue. The energy of released electrons is controlled with the incident light, such that the process can be investigated for a broad range of energies and for various dielectrics. "Every interaction of high-energy radiation with tissue results in the generation of electrons. These in turn transfer their energy via inelastic collisions onto atoms and molecules of the tissue, which can destroy it. Detailed insight about electron scattering is therefore relevant for the treatment of tumors. It can be used in computer simulations to optimize the destruction of tumors in radiotherapy while sparing healthy tissue," highlights Professor Matthias Kling of the impact of the work. As a next step, the scientists plan to replace the glass nanoparticles with water droplets to study the interaction of electrons with the very substance which makes up the largest part of living tissue.

Van Den Nest M.,Max Planck Institute of Quantum Optics
Physical Review Letters | Year: 2013

We show that universal quantum computation can be achieved in the standard pure-state circuit model while the entanglement entropy of every bipartition is small in each step of the computation. The entanglement entropy required for large-scale quantum computation even tends to zero. Moreover we show that the same conclusion applies to many entanglement measures commonly used in the literature. This includes e.g., the geometric measure, localizable entanglement, multipartite concurrence, squashed entanglement, witness-based measures, and more generally any entanglement measure which is continuous in a certain natural sense. These results demonstrate that many entanglement measures are unsuitable tools to assess the power of quantum computers. © 2013 American Physical Society.

Tu H.-H.,Max Planck Institute of Quantum Optics
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

We propose a class of projected BCS wave functions and derive their parent spin Hamiltonians. These wave functions can be formulated as infinite matrix product states constructed by chiral correlators of Majorana fermions. In one dimension, the spin Hamiltonians can be viewed as SO(n) generalizations of Haldane-Shastry models. We numerically compute the spin-spin correlation functions and Rényi entropies for n=5 and 6. Together with the results for n=3 and 4, we conclude that these states are critical and their low-energy effective theory is the SO(n)1 Wess-Zumino-Witten model. In two dimensions, we show that the projected BCS states are chiral spin liquids, which support non-Abelian anyons for odd n and Abelian anyons for even n. © 2013 American Physical Society.

Reiserer A.,Technical University of Delft | Rempe G.,Max Planck Institute of Quantum Optics
Reviews of Modern Physics | Year: 2015

Distributed quantum networks will allow users to perform tasks and to interact in ways which are not possible with present-day technology. Their implementation is a key challenge for quantum science and requires the development of stationary quantum nodes that can send and receive as well as store and process quantum information locally. The nodes are connected by quantum channels for flying information carriers, i.e., photons. These channels serve both to directly exchange quantum information between nodes and to distribute entanglement over the whole network. In order to scale such networks to many particles and long distances, an efficient interface between the nodes and the channels is required. This article describes the cavity-based approach to this goal, with an emphasis on experimental systems in which single atoms are trapped in and coupled to optical resonators. Besides being conceptually appealing, this approach is promising for quantum networks on larger scales, as it gives access to long qubit coherence times and high light-matter coupling efficiencies. Thus, it allows one to generate entangled photons on the push of a button, to reversibly map the quantum state of a photon onto an atom, to transfer and teleport quantum states between remote atoms, to entangle distant atoms, to detect optical photons nondestructively, to perform entangling quantum gates between an atom and one or several photons, and even provides a route toward efficient heralded quantum memories for future repeaters. The presented general protocols and the identification of key parameters are applicable to other experimental systems. © 2015 American Physical Society.

Romero-Isart O.,Max Planck Institute of Quantum Optics
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

We analyze the requirements to test some of the most paradigmatic collapse models with a protocol that prepares quantum superpositions of massive objects. This consists of coherently expanding the wave function of a ground-state-cooled mechanical resonator, performing a squared position measurement that acts as a double slit, and observing interference after further evolution. The analysis is performed in a general framework and takes into account only unavoidable sources of decoherence: blackbody radiation and scattering of environmental particles. We also discuss the limitations imposed by the experimental implementation of this protocol using cavity quantum optomechanics with levitating dielectric nanospheres. © 2011 American Physical Society.

Karshenboim S.G.,Max Planck Institute of Quantum Optics
Physical Review Letters | Year: 2010

Constraint on spin-dependent and spin-independent Yukawa potential at atomic scale is developed. That covers constraints on a coupling constant of an additional photon γ* and a pseudovector boson. The mass range considered is from 1eV/c2 to 1MeV/c2. The strongest constraint on a coupling constant α′ is at the level of a few parts in 1013 (for γ*) and below one part in 1016 (for a pseudovector) corresponding to mass below 1keV/c2. The constraints are derived from low-energy tests of quantum electrodynamics and are based on spectroscopic data on light hydrogenlike atoms and experiments with magnetic moments of leptons and light nuclei. © 2010 The American Physical Society.

Mezzacapo F.,Max Planck Institute of Quantum Optics
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We study the ground-state phase diagram of the quantum J 1-J 2 model on a square lattice by means of an entangled-plaquette variational Ansatz. In the range 0≤J 2/J 1≤1, we find classical magnetic order of Néel and collinear type, for J 2/J 1 0.5 and J 2/J 1 0.6, respectively. For intermediate values of J 2/J 1 the ground state is a spin liquid (i.e., paramagnetic with no valence-bond crystalline order). Our estimates of the entanglement entropy show that such a spin liquid is topological. © 2012 American Physical Society.

Wu H.-C.,Los Alamos National Laboratory | Meyer-Ter-Vehn J.,Max Planck Institute of Quantum Optics
Nature Photonics | Year: 2012

Half-cycle picosecond pulses have been produced from thin photoconductors when applying an electric field across the surface and switching on conduction using a short laser pulse. The transverse current in the wafer plane then emits half-cycle pulses in a normal direction, and pulses of 500fs duration and 1×106V m-1 peak electric field have been observed. Here, we show that single half-cycle pulses with a duration of 50Â as and up to 1×1013V m-1 can be produced when irradiating a double foil target with intense few-cycle laser pulses. Focused onto an ultrathin foil, all electrons are blown out, forming a uniform sheet of relativistic electrons. A second layer, placed some distance behind, reflects the drive beam but lets electrons pass straight through. Under oblique incidence, beam reflection provides the transverse current, which emits intense half-cycle pulses. Such a pulse may completely ionize even heavier atoms. With these developments, new types of attosecond pump-probe experiments will become possible. © 2012 Macmillan Publishers Limited. All rights reserved.

Orus R.,Max Planck Institute of Quantum Optics
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

In this paper we explore the practical use of the corner transfer matrix and its higher-dimensional generalization, the corner tensor, to develop tensor network algorithms for the classical simulation of quantum lattice systems of infinite size. This exploration is done mainly in one and two spatial dimensions (1D and 2D). We describe a number of numerical algorithms based on corner matrices and tensors to approximate different ground-state properties of these systems. The proposed methods also make use of matrix product operators and projected entangled pair operators and naturally preserve spatial symmetries of the system such as translation invariance. In order to assess the validity of our algorithms, we provide preliminary benchmarking calculations for the spin-1/2 quantum Ising model in a transverse field in both 1D and 2D. Our methods are a plausible alternative to other well-established tensor network approaches such as iDMRG and iTEBD in 1D, and iPEPS and TERG in 2D. The computational complexity of the proposed algorithms is also considered and, in 2D, important differences are found depending on the chosen simulation scheme. We also discuss further possibilities, such as 3D quantum lattice systems, periodic boundary conditions, and real-time evolution. This discussion leads us to reinterpret the standard iTEBD and iPEPS algorithms in terms of corner transfer matrices and corner tensors. Our paper also offers a perspective on many properties of the corner transfer matrix and its higher-dimensional generalizations in the light of novel tensor network methods. © 2012 American Physical Society.

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